Jupiter's red spot is actually a giant storm,
similar to hurricanes on Earth, but much, much
larger. The storm is actually over twice the size
of Earth! Why is the spot red? Some scientists
suggest that it is because of certain phosphorous
containing compounds in the gases that comprise
the storm, but the spot's color is not well
understood.

Answer 2:

Jupiter’s red spot is a storm, probably similar
(but much larger) than a hurricane on Earth. How
it formed, and why it's still there after over 300
years, we don't yet know. Saturn and Neptune have
similar storms, so it's a common thing for gas
giant planets to have such storms.